Field
The present disclosure relates generally to coaxial terminations used to terminate ports that are adapted to receive coaxial cable connectors, and more particularly, to an improved coaxial termination that offers enhanced protection against repeated high-voltage surges.
Technical Background
RF coaxial cable systems are used in the cable television industry for distributing radio frequency signals to subscribers of cable television service, and more recently, voice and data telecommunications services. The coaxial cables used to route such signals include a center conductor for transmitting a radio frequency signal, and a surrounding, grounded outer conductive braid or sheath. Typically, the coaxial cable includes a dielectric material surrounding the center conductor and spacing it from the grounded outer sheath. The diameter of the center conductor, and the diameter of the outer conductor, and type of dielectric are selected to produce a characteristic impedance, such as 75 ohms, in the coaxial line. This same coaxial cable is sometimes used to provide AC power (typically 60-90 Vrms) to the equipment boxes that require external power to function.
Within such coaxial cable systems, such coaxial lines are typically coupled at their ends to equipment boxes, such as signal splitters, amplifiers, etc. These equipment boxes often have several internally-threaded coaxial ports adapted to receive end connectors of coaxial cables. If one or more of such coaxial ports is to be left “open”, i.e., a coaxial cable is not going to be secured to such port, then it is necessary to “terminate” such port with a coaxial termination that matches the characteristic impedance of the coaxial line (e.g., a 75 ohm termination). If such a coaxial termination is omitted, then undesired reflected signals interfere with the proper transmission of the desired radio frequency signal.
When deployed in the field, as in cable TV systems, for example, these known coaxial termination devices can be subjected to power surges caused by lightning strikes and other events. These power surges can damage or destroy the resistive and/or capacitive elements in such a termination, rendering it non-functional.
An older specified surge test, ANSI C62.41 Category B3, specified that a 6,000 Volt open circuit/3,000 Amp short circuit surge pulse be injected into the coaxial termination device. At least some of the known coaxial termination devices have difficulty complying with such surge test. Indeed, efforts to make the resistive and capacitive components larger, in order to withstand such power surges, can have the negative impacts of increased costs and/or creating a larger impedance mismatch, and hence, causing poorer levels of RF Return Loss performance. One approach to designing a termination that can withstand the previously mentioned 6,000 Volt surges would be to use a 6,000 Volt capacitor and a high power resistor. Unfortunately, such components are relatively expensive and have a much larger physical size, which tends to increase the size and cost of the housing necessary to contain such components, thereby resulting in a much bulkier and more costly design. In more recent times, a newer surge test (ANSI/SCTE 81 2012) has been introduced by the industry requiring a different test profile as summarized in table 1 below. Older designs such as that related in U.S. Pat. No. 6,751,081 (Kooiman) exhibit severe Return Loss degradation after subjection to this newer surge test profile.